1. Field of the Invention
The present invention relates to a battery pack which detects the residual capacity of a secondary battery.
2. Description of the Related Art
In recent years, a high-performance secondary battery has been developed which can supply an electric current of 100 A or above even at a capacity of several ampere-hours. Such a secondary battery has been noticed as the power source of an apparatus which bears a heavy load and is also subjected to considerable variations in the load, such as an electric tool, an electrically-driven bicycle and a hybrid electric vehicle. Besides, this secondary battery is frequently used at a great electric current. Therefore, it is essential to detect its residual capacity so that the secondary battery can be prevented from being damaged and its safety can be secured.
As the method of detecting the residual capacity of a secondary battery, a current accumulation method and a voltage monitoring method have been known. The current accumulation method is a method in which the electric current which flows through a secondary battery is accumulated so that its residual capacity can be calculated. On the other hand, the voltage monitoring method is a method where the voltage of a secondary battery is measured, and based on the measured voltage, the residual capacity can be calculated.
The current accumulation method has an advantage in that even if the load varies largely, the residual capacity can be detected more precisely than the voltage monitoring method.
In contrast, the voltage monitoring method is advantageous in that even if the load varies slightly, the residual capacity can be detected more accurately than the current accumulation method.
In general, therefore, if a load apparatus whose load is not varied greatly is connected to a secondary battery, its residual capacity can be detected by the voltage monitoring method. On the other hand, if a load apparatus whose load is largely varied is connected to a secondary battery, the residual capacity can be detected by the current accumulation method.
Furthermore, in WO No. 98/56059 specification, a battery pack is proposed in which the method is switched from the current accumulation method to the voltage monitoring method, and vice versa, according to an electric current. Specifically, the electric current which passes through a secondary battery is measured, and if the measured electric current is large, the residual capacity is calculated by the current accumulation method. In contrast, if the measured electric current is not large, the residual capacity is calculated by the voltage monitoring method.
However, in the battery pack according to WO No. 98/56059 specification, the current accumulation method is used when the electric current is large. But if the above described apparatus which requires a great electric current, such as an electric tool, is connected to this battery pack, then the battery pack needs to be formed by a current sensor which can detect a large electric current. Such a great-current detectable current sensor is generally expensive, thus raising costs for a battery pack.